OCR Computer Science GCSE Paper 1 Notes:
These notes provide a thorough and organized resource to prepare for OCR's Computer Science GCSE Paper 1. This paper assesses your knowledge and understanding of core computer science principles and practices. The topics covered in these notes include:
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Clock Speed:
The speed a processor operates is quoted as clock speed. It is measured as hertz (Hz). It is the
number of cycles per second or the rate at which the electrical current changes in the circuit.
The 1’s turn to 0’s and the 0’s turn to 1’s. Everything the processor does occurs on the “tick” of
the clock. A faster clock means more instructions are fetched, decoded and executed in a
second.
One clock cycle = 1 Hz. clock speed is measured in gigahertz (GHz), one billion cycles per
second. Typical speeds are between 2-4 GHz.
In theory, a computer with a 4GHz processor should be twice as fast as one with 2 GHz. But
because there are other components that contribute to the overall speed this is not true. Each
one creates a bottleneck.
Cache memory:
One bottleneck that can occur is the access speed of the main memory, known as random
access memory. Reading from and writing to main memory is much slower than the speed at
which the processor can work. The logical answer is to use faster memory technologies, but
this increases the price of the computer.
Modern computers need to run many programs at the same time, so they need lots of memory.
There needs to be a compromise between speed and cost.
One way of improving the speed at minimal cost is to use a small amount of cache, much
faster, but more costly, memory where frequently used instructions or data can be stored
temporarily.
Cache memory is an intermediary between the main memory and the CPU. The cache makes
any data frequently used by the CPU available much more quickly. Because the processor has
to access main memory less often, it can work faster, so the CPU performance increases. If the
required information is not located in the cache it has to be fetched from ram.
A typical PC might have 8 or 16 GB of ram, but only 2 MB of the faster more expensive cache
memory. Notice the different units here and remember that there are 1000 megabytes in a
gigabyte. This computer, therefore, has thousands of times more RAM than cache memory.
The more cache memory a computer has, the more data and instructions can be held in the
cache and made available very quickly. This improves processor performance.
There are different levels of cache:
● Level 1 cache is extremely fast but small (2-64KB). Thus speeding up the fetch-execute
cycle
● Level 2 cache is relatively fast (much faster than ram) and medium-sized (256KB-2MB)
● Some CPUs also have level 3 data cache
,Number of Cores:
When we looked at the basic Fetch-Decode-Execute cycle we assumed that there was a single
processor and a single main memory. You have probably heard the terms dual-core and
quad-core.
Today’s more complex CPUs typically include more than one core. A dual-core processor has
two processing units within the CPU and a quad-core has four. This means that parallel
processing can take place, with two or four instructions being executed simultaneously. In
theory, having two cores means that the computer can perform twice as many operations per
second but this isn't always the case.
A program is a series of instructions that need to be in order. Multiple cores could work on
different programs that operate in parallel, but a quad-core processor isn't necessarily four
times faster unless the computer is designed to use multiple cores. However, a PC with a
multi-core processor, executing many tasks at the same time, will operate faster than a
single-core processor.
Fetch-Decode-Execute Cycle:
Fetch:
1. PC contains address of next instruction to be executed
2. Contents of PC copied to MDR
3. Contents of MAR passed along address bus to RAM
4. Read signal sent from CU to MAR
5. Instruction from specified location is copied to data bus
6. Going to MDR
7. The PC is incremented
8. Contents in MDR put into CIR (Current Instruction Register)
Registers:
Mar:
● Stores the address/location where data will be read/written/accessed/fetched
, ● Address/location of data/instruction being processed
MDR:
● Stores the data/instruction that is fetched/read from memory
● Stores the data/instruction that is to be written to memory
● Stores the data/instruction from the address in MAR
● data/instruction next to be processed
PC
● Stores the address/location of the next instruction to be processed
● Stores the address/location of the current instruction being run
Embedded system:
An embedded system is a small computer built into a piece of equipment designed to perform a
specific function. Examples of items that use this technology include vehicles, cameras,
medical equipment, aircraft, vending machines, ovens, fridges, mobile phones, satellite
navigation devices, televisions, digital clocks and lifts.
System Purpose
Microwave The computer system inputs heat into the microwave for a
selected time to warm up food.
Traffic light The computer system waits for a certain amount of time after the
button is pressed and then changes the light from green to
amber to red
Car When the engine is turned on, the computer system controls the
amount of fuel added to the engine.
Lift The computer system moves the lift to the right floor when
someone presses a button to go to a different floor
Vending machine When someone inputs a code into the vending machine, the
computer system determines which snack is going to fall, then
waits for the money, then drops the snack so the person is able
to pick it up..
Characteristics of an embedded system:
The main characteristics of an embedded system are reliability and minimal resources such as
ROM, timers, sensors and actuators.
The program which controls equipment is held in ROM and cannot be changed. It is typically
written in a machine-efficient language so that it uses the minimum amount of memory and
executes as fast as possible. Assembly language may be used for some critical parts of the
program.
It has a very limited operating system. Unlike the operating system in a desktop computer, the
embedded operating system does not have to be loaded when the machine is switched on, as
it is held in ROM. The operating system is only able to run a single application.
, It has a very simple user interface, such as the buttons on a microwave oven or washing
machine. Some embedded systems have no user interface.
Some embedded systems have sensors designed to measure external stimuli and react
accordingly. For example, a central heating system may use buttons and sliders to make the
heating come on at certain times of day, on different days, and maintain a chosen temperature.
Characteristics of embedded systems:
● Low power consumption
● Cheap to mass produce
● Rugged
● Limited or no programmable functions
● Use dedicated software that has already been set up (so the user does not need to
install it)
1. An embedded system is made up of a microcontroller (SoC). Name three things you
might find on a microcontroller.
CPU, RAM, I/O
2. What is firmware made up of?
OS and software
3. Where is firmware stored?
ROM
4. Give an example of an embedded system.
Lift
1. What are the differences between an embedded system and a general-purpose
computer?
An embedded system is one that has been designed for a specific function and is
restricted to only a few tasks. A general-purpose computer is able to perform many
different tasks and is not designed for a specific function.
2. Give three examples of an embedded system in your house describing the purpose of
the device it is embedded into and the input–process–output of the embedded machine.
Toaster - To heat up bread - Input: light, medium, burnt - processing: calculate the
temperature and time of heating element - output: Rack goes up.
Washing machine - to wash clothes - input: type of wash required - processing:
calculate the time of washing and amount of water and soap to put into the washing
machine - output: clean clothes.
Microwave - to heat up food - Input: type of warming required and time - Processing:
heat required - output: warm food.
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